Orbiting Astronomical Observatory OAO-2

The Orbiting Astronomical Observatory-2 (OAO-2) was launched
from Cape Kennedy into a 480-nautical mile circular orbit on 1968
December 7 to carry out astronomical observations in the far-ultraviolet
(UV). The spacecraft was launched by an Atlas-Centaur vehicle.
The instrument section contained the Smithsonian Astrophysical
Observatory telescopes in one end and the Wisconsin Experiment
Package (WEP) looking out the other direction. The WEP consisted
of five filter photometers for measurements in the wavelength
interval 1330 Å - 4250 Å and two scanning spectrometers
covering the region from 1050 Å to 3600 Å. OAO-2 was
the first true observatory in space and was in operation from
December 1968 to January 1973. The spacecraft provided power,
communication, spacecraft orientation, and pointing to approximately
1 minute of arc with a stability of the order of 1 arc second,
storage of commands to be performed, and storage of experimental
data for later transmission to the ground.

The Wisconsin instrumentation observed over 1200 objects in
the UV for the first time, including planets, comets, a variety
of stars, star clusters, and galaxies. Among OAO-2 results are
the discovery that comets are surrounded by huge hydrogen halos;
evidence that at least some novae increase their UV brightness
at the same time that their visible light is fading rapidly; and
that galaxies are systematically brighter in the UV than expected
from the visual colors of stars which make them up. OAO data were
used to investigate the physical properties of interstellar dust
and to map the distribution of hydrogen near the sun; OAO data
combined with measurements of angular diameters of stars enabled
the first empirical determinations of the temperatures of the
hotter stars.